The present invention relates to a gradiometer sensor which employs at least three vector magnetometers to measure a magnetic field gradient. More particularly. the present invention relates to a three SQUID (i.e., Superconducting Quantum Interference Device) gradiometer.
A description of a SQUID device used to sense magnetic fields is given below based on the SQUID device shown in FIG. 1. SQUID 1 comprises a superconducting loop 2 having at least one weak link (e.g., Josephson device J) which can exhibit a Josephson current. SQUID 1 is located near a SQUID input coil 3 which is electrically connected to a pick-up coil 4.
When a change in the magnetic field to be detected occurs through pick-up coil 4, a circulating current .DELTA.i will be induced in the SQUID input coil 3. Circulating current .DELTA.i produces a magnetic field which couples to the SQUID loop 2 and is detected. Pick-up coil 4 has an inductance L.sub.U which is approximately equal to an inductance L.sub.i of input coil 3. The inductance L.sub.p of the connecting line between input coil 3 and pick-up coil 4 (i.e., the parasitic inductance) should be very small (i.e., L.sub.p much less than L.sub.i). This can be accomplished, for example, as discussed in IBM Technical Disclosure Bulletin, Volume 27 No. 5, October 1984, pages 2822-2823.
FIG. 2 is a graph of voltage vs. modulation current illustrating hysteresis present in a single SQUID device such as the one shown in FIG. 1 (at T=14.degree. K. and at a field sweep frequency of approximately 13 Hz).
The SQUID device shown in FIG. 1 is referred to as an unlocked SQUID because there is no feedback to cancel out the background field. FIG. 3 is a graph of hysteresis (.phi..sub.0) vs. field sweep amplitude (.phi..sub.0 p-p) of an unlocked SQUID at T=77.degree. K. FIG. 3 illustrates that as the sweep of the applied magnetic field increases, the hysteresis of the unlocked single SQUID device will also increase.
Therefore, SQUIDs have been "locked up" by providing feedback in order to prevent hysteresis. This is accomplished by providing feedback so that the value of the magnetic field which the SQUID sees is kept constant (i.e., the SQUID never sees a change in the magnetic field). A change in magnetic field produces a correction current which in turn produces an equal and opposite field. Such an arrangement of "locking up" SQUIDs is illustrated in FIG. 4.
A magnetic field gradient may be measured by a device which uses the output of two magnetometers separated at a distance d apart from each other. FIG. 4 illustrates such a magnetic field gradient measuring device (i.e., gradiometer). The gradiometer illustrated in FIG. 4 is a two SQUID gradiometer (known in the art as a Bare SQUID Gradiometer). Each SQUID 6a and 6b measures the magnetic field at its respective location. Amplifiers 9a, 9b, feedback coils 7a, 7b and resistors 8a, 8b (each having the same resistance R.sub.F) are used to provide a correction current producing a field equal to and opposite that of the magnetic field Electronic voltages 'B.sub.L ' and 'B.sub.R ' are supplied as outputs corresponding to the magnetic field at each of SQUIDs 6a and 6b, respectively. The difference between these outputs of SQUIDs 6a and 6b is taken electronically to form the gradient. Thus, the gradient is equal to: ##EQU1##
Such a two SQUID gradiometer is easy to make, has good balance and low hysteresis. However this type of gradiometer is seldom used because the large common mode signal of the two magnetometers (from the non-gradient terms in the magnetic field) requires an almost impossible degree of common mode rejection (1 part in 10.sup.9) of an amplifier taking the difference in the outputs between the two SQUIDs. That is, the gradiometer of FIG. 4 is virtually impossible to operate due to the difficulty of electrically detecting a small gradient in the presence of a very large background magnetic field due to the earth's magnetic field. The gradiometer of FIG. 4 attempts to subtract two very large numbers to provide a relatively very small number as the gradient, for example, a ratio of the gradient to the background field of approximately 1:10.sup.9.
The electronics associated with such a gradiometer must detect the difference between two magnetic fields where the average background field is very large. This requires the electronics to detect an extremely small signal difference in the presence of a very large signal, which is very difficult and very costly.
FIG. 5 illustrates a single SQUID thin film gradiometer. Reference numbers 111 and 112 each represent thin film pick-up coils. Reference numeral 113 represents the input coil and reference numeral 114 represents a washer-type SQUID. The thin film pick-up coils 111 and 112 each intercept the magnetic field at their respective locations to determine the gradient ##EQU2##
The currents produced in each of the thin film pick-up coils 111 and 112 are opposite to each other. Therefore, SQUID 114 operates as a null detector. That is, when the SQUID 114 output is equal to zero, there is no gradient. The SQUID 114 output is therefore proportional to the gradient ##EQU3##
A thin film gradiometer such as the one illustrated in FIG. 5 is easy to make and has a very well known design when fabricated from low T.sub.c superconductive materials. However, when it is fabricated from high T.sub.c superconductive materials, the FIG. 5 gradiometer exhibits excessive hysteresis and is very hard to make due to the difficulties associated with high T.sub.c thin film crossover and planar input coil 113. Therefore, it is not advantageous to use the thin film gradiometer of FIG. 5 when fabrication is made using high T.sub.c superconductive materials.